1. Field of the Invention
This invention pertains generally to charge storage devices and methods of manufacture and more particularly to charge storage devices containing thin films of nano-scale oxide materials that are mesoporous and methods of fabrication.
2. Description of Related Art
In traditional charge storage devices, there is a trade off between energy density and power density. Batteries and fuel cells offer high energy density but low power density. Capacitors, such as dielectric and electrolytic capacitors used in electronic circuits, offer high power density but have a low energy density.
Capacitive energy storage has been somewhat overlooked as an energy storage technology. The technology is based on electrochemical capacitors (ECs), which include double layer capacitors (also termed supercapacitors or ultracapacitors) and pseudocapacitors. A double layer supercapacitor (DLS) stores energy in the interface between the electrode and the electrolyte without chemical reactions taking place. Electrochemical supercapacitors (ECS) undergo electron transfer reactions during charge and discharge, and store energy through faradaic reaction processes. Pseudocapacitors represent a class of charge storage materials that undergo redox reactions like a battery material and yet respond in a capacitive fashion. With these materials, there is the prospect of maintaining the high energy density of batteries without compromising the high power density of capacitors.
One of the limiting features that prevents more widespread usage of electrochemical supercapacitors has been the relatively low energy density of the materials employed in capacitive storage applications. Currently, the field is largely based on materials made of carbon and electric double layer storage processes. A significant phenomenon occurs as electroactive materials approach nanometer-scale dimensions. The charge storage of cations from faradaic processes occurring at the surface of the material, referred to as the pseudocapacitive effect, becomes increasingly important.
Thus, in recent years, there has been considerable effort aimed at increasing specific energy without compromising specific power. One particularly popular direction has been the study of carbon materials with tailored pore sizes. An interesting development in this area is the report of an anomalous increase in capacitance for pore diameters below 1 nm.
Interestingly, continuing to increase the surface area of the carbon does not necessarily lead to increasing specific capacitance as limiting values have been observed experimentally and modeled as well. Although existing electric double layer capacitors (EDLC's) have high power capabilities, the energy density for electric double layer capacitors is well below that of batteries. The reason is that only the specific surface area of the carbon electrode contributes to energy storage while in the battery, the entire material, surface and bulk, contributes to the storage capacity.
Metal oxides are another grouping of materials that have been studied for use in electrochemical capacitors. The interest in using pseudocapacitor based materials for electrochemical capacitors is that the energy density associated with faradaic reactions is theoretically much higher, by at least an order of magnitude, than traditional double layer capacitance.
For example, hydrous ruthenium oxide (RuO2) has been shown to have a specific capacitance greater than 700 F/g. Interestingly, the highest values of specific capacitance do not occur with anhydrous crystalline (RuO2). In the anhydrous material, the redox activity is confined to the electrode surface via adsorption of protons from solution, and specific capacitances of 380 F/g have been reported. Instead, the largest capacitive storage has been observed with the hydrous amorphous phase of RuO2 as there is now the additional contribution of proton insertion into the bulk as well as the surface redox reactions. Specific capacitances with these ruthenium materials reach 760 F/g, but only at scan rates on the order of 2 mV/s. Thus, the energy density of hydrous (RuO2) may be somewhat less attractive because the highest levels are achieved at the expense of a lower power density. Furthermore, there has been considerable interest in moving away from (RuO2) not only because of cost issues, but also to use more environmentally friendly electrolytes than concentrated sulfuric acid.
Other transition metal oxide systems exhibit analogous behavior. For the most part, however, the various transition metal oxide systems exhibit specific capacitances in the range of 200 to 250 F/g, i.e., only 10 to 20% of their theoretical values.
Accordingly, there is a need for materials that achieve a combination of high power and energy density that are capable of multiple charge/discharge cycles that are easy to manufacture and use. There is also a need for simple fabrication methods and electrode systems. The present constructs and methods satisfy these needs, as well as others, and are generally an improvement over the art.